The present application relates indirectly to measuring the gas concentration in a gas blend or mixture through directly measuring the thermal conductivity of that blend or mixture.
Generally, it is known to use differences in thermal conductivity, e.g different thermal conductivities of gases, to determine the consistency, constituents, and components of the gas. This approach that the thermal conductivity be measured at a particular gas temperature, or at least at an average temperature in those instances in which the temperature is, e.g. modulated, for reasons of signal processing. Only binary blends can be determined as to their components under such conditions. In case another component, be it a known one or unknown will falsify the measurement signal.
For measuring thermal conductivity by means of analyzers the following principles are being used at this time. A resistive heater is uses as a source for thermal energy, and by means of a particular electric current that element has it temperature raised vis-a-vis the respective environment. Heat is extracted through a heat conductive path of well defined geometry, and that conductive path is used to have heat transferred from that source to a heat sink of known temperature and under traversal of a particular path length of and within that gas. The heat transport from source to sink extracts a certain amount of energy from the source, and that flow and the resulting temperature conditions are used as an indicator and representation for the thermal conductivity of the gas occupying the space between source and sink. The heat is measured with an appropriate known method.
It is well known that the thermal conductivity of a gas is not constant but depends on its temperature. The dependence must be excluded through calibration and constancy conditions. The measuring cells are themselves subject to temperature control through an electronic circuit so that the measuring cells are being kept at a constant temperature. In addition, the average gas temperature is determined so that the temperature of the source itself should remain constant or at least reproducible.
A problem is posed by the conventional thermal conductivity sensing devices in that, on one hand, the particular path of the heat flow must be passed through continuously by gas. Also, a rather quick gas exchange is needed, and the flow of the gas, of course, determines but also interferes with the thermal transport, so that there is another parameter which provides possible interference with the measuring result.
German printed patent application 35 02 440 describes a method for compensating the measurement for the gas flow. German patent 29 52 137 describes a measuring cell wherein the gas is not subject to flow into and out of the area of measurement, but there is exchange through diffusion. In this case, the volume for the chamber in the gas path is relatively large so that the gas exchange in the measuring chamber may be too slow for certain tasks.